The
economic cost of erosion on the eroded site itself can be
expressed in a variety of terms.

LOSSES IN
PRODUCTIVE LAND AND OTHER OBSTACLE TO DEVELOPMENT

Taking the
example of the Salci pineapple plantation at Ono in southern
Côte d'Ivoire, land clearance, followed by mechanized farming of
a 1000-ha industrial plantation very quickly brought erosion
problems, which were countered by installing access tracks along
contour lines, establishing grass-covered ridges and alternating
rows of pineapples of different ages and ground-covering
capacities: it takes six months for a pineapple to cover more
than 90% of the soil and protect it against erosion. However, in
about 1973, the importation of new mechanized cropping techniques
from Hawaii, based on a tanker with a 17-m sprayer arm for
fertilizer, weed-killer and nematicide meant a further redesign
of the plantation, eliminating the banks and contour tracks to
allow for 34-m wide planted rows more or less following the
contour lines. Gullying erosion appeared at once, preventing the
heavy machinery from reaching whole sectors, which therefore had
to be worked once more by hand (which was not very
cost-effective). The proportion of productive land lost to
erosion (gullying, plants uprooted, the burying of plants under a
layer of sterile sand, etc.) amounted to barely 2%, which is a
more than acceptable figure. However, the affected area that had
to be abandoned for mechanization was much greater (70 ha for one
large gully alone) and production delays increased every year.
Once channels have formed, runoff water generally follows the
same route, so that soil loss increases over time. It is
interesting to note that 1000 ha of small plots cleared and
manually farmed by small African planters have never had any
erosion problem. The use of heavy vehicles reduces soil
infiltration capacity and increases runoff, which collects on
tracks before creating gullies on the plots. In tropical areas
erosion takes effect very fast - about two to four years -
whereas it takes 30 years for similar effects of excessive
mechanization to be observed in Europe.

Another
example. In England, Evans (1981) studied erosion on a 10 ×
10 km area north of London. Here again, the actual area effected
by erosion was small (2.9%) but concentrated in certain points:
the steep slopes bordering the loess uplands and farmed by the
poorest farmers, who could not afford to rest the land or put it
under permanent pasture, since they have to ensure food
self-sufficiency or at least a corresponding cash income. The
potential risks of erosion on these steep slopes and of damage in
the case of heavy rainstorms are much greater than on the
plateaux, which belong mostly to rich landowners. There may
therefore be a connection between erosion risks, the social and
economic level of small farmers, fragile land, and the interest
of such farmers in erosion control.

LOSSES IN
YIELD AND PROFIT MARGINS

Although
production losses (2 to 5%) may be slight and easily compensated
for in regional terms through the use of new inputs (fertilizers,
drainage, mechanization of tillage), the situation is very
different for individual small farmers. As much as 10% of topsoil
can be lost on this steeply sloping land, with a 30% fall in
production and a 50% fall in net income once inputs have been
paid for, so that the profit margin essential for the farmer's
family shrinks seriously. Erosion therefore has a greater effect
on small farmers, who will be marginalized for lack of credit
facilities, initiative or know-how. And they can do nothing about
it without a radical change in production methods (high-return
production). So there is spiralling impoverishment for the poor
and a search for new solutions by those who have the means.

VARYING
EFFECTS OF EROSION ON SOIL PRODUCTIVITY

In the
United States the cost of the Soil Conservation Service and its
slight impact on soil loss led people to question the effects of
erosion on soil productivity (the basis of Bennett's SWC system).
It was observed that productivity had hardly fallen at all on
deep loess soils, which are homogenous to a depth of several
metres; indeed, the ill effects of erosion (-1% in yields) were
easily made up for by providing new inputs (Dregne 1988) (cf.
Figure 6). On the other hand, thin rendzines (forest soils where
fertility is concentrated in the topsoil) and many tropical soils
very quickly lose their productive capacity.

SELECTIVE
EROSION OF FINE PARTICLES, NUTRIENTS AND ORGANIC MATTER

If the
quality of eroded soil and the runoff water collected downstream
of the eroded plots is compared with the soil left in place to a
depth of 10 cm, in terms of plant cover and extent of losses
through erosion (Table 3), the following results are seen (Roose
1977a):

 nutrient
losses grow in parallel with the volume of runoff and eroded
matter; but the nutrient content of soil falls more slowly
than the rise in the volume of displaced soil and water;


much higher proportions of nutrients are found in the water
and eroded soil than in the soil in place (horizon: 10 cm);
this is clear for carbon, nitrogen, phosphorus, clay and loam
(up to 50 m) but still more striking for
exchangeable bases (14 to 18 times more on cropped land);
sheet erosion is thus selective in terms of the nutrients and
colloids that are the essence of soil fertility;


the lower the eroded volume the greater the selectivity of
sheet erosion, i.e. from bare to cropped and from cropped to
forest land.

This is
easily explained in two ways (Figure 7). On the one hand, the
removal capacity of sheet runoff is slight since it is slowed
down by the roughness of the soil surface, stalks, exposed roots
and litter. Sheet runoff can remove only the light matter: the
organic matter, clays and loams to which most nutrients are
bound. Forest soils, on the other hand - and to a lesser extent
savannah soils - accumulate organic matter and nutrients on the
surface. When the rain beats down on these soils the first few
millimetres - the richest - are the first to be eroded. The more
erosion advances, the more it comes from rills and gullies, and
the more the generally poorer subsurface is involved. Scoured
land is thus less selectively enriched than in the case of
surface sheet erosion.

THE COST OF
NUTRIENT LOSS

Another
aspect of economic loss from erosion is the amount and cost of
fertilizer needed to replace the nutrients lost through erosion.
This has been calculated by Roose (1973 and 1977a) for southern
Côte d'Ivoire. Under secondary rainforest, chemical losses from
erosion are slight: 26 kg/ha/yr of carbon + 3.5 kg of nitrogen +
0.5 kg of phosphorus, and a few kg/ha/yr of bases. These losses
are easily made up through biological upwelling (OM deposits in
litter) and the nutrients in rainwater.

TABLE 3Selective losses from
sheet erosion on a 7% slope at (Adiopodoumé Côte d'Ivoire) as a
function of plant cover (cf. Roose, 1977a)

Total erosion
(kg/ha/yr)

Selectivity factor as
a function of soil in place (10 cm)

Forest

Crop

Bare soil

Forest

Crop

Bare soil

Total carbon

26.4

855.6

2725

12.8

2.1

1.5

Total nitrogen

3.5

98.3

259

22.5

3.1

1.9

Total phosphorus

0.5

28.5

111

6.6

1.4

1.3

Exchangeable CaO

3.0

49.9

113

492

18.5

9.7

Exchangeable MgO

2.2

29.0

45

327

14.1

5.1

Exchangeable K2O

1.2

17.7

35

550

2.4

1.1

Exchangeable Na2O

0.6

9.5

15

849

15.4

5.6

Total CaO

3.7

57.1

139

216

8.8

5.0

Total MgO

2.3

39.0

78

60

5.8

2.7

Total K2O

1.3

35.1

87

18

1.7

1.0

Total Na2O

0.6

12.6

27

49

3.2

1.6

Clay 0-2 microns

64.5

5142

18275

5.9

1.2

1.1

1.5

1.2

Loam 2-50 microns

33.8

2179

7115

7.7

2.5

1.9

Fine sand 50-200 microns

1.7

5174

-23135

0.1

0.6

0.6

Coarse sand 200-2000

0

19305

89375

0

0.9

0.9

1.1

1.2

Total erosion t/ha

0.11

32

138

Runoff m³/ha

210

5250

6300

 The first
observation is that the total eroded products are less than
the erosion indicated at the bottom of the table, inasmuch as
the amounts of Fe2O3, Al2O3
and SiO2 (clays) have not been counted - nor, more
especially, that of the residue insoluble in triacide (quartz
sands).


The increase in chemical losses is almost parallel with that
of soil losses, and is thus in inverse proportion to plant
cover. Nutrient concentrations in the eroded substances
decrease somewhat as erosion increases, but this decrease is
not proportionate to the increase in water and soil losses.


Since erosion has short- and medium-term repercussions, a
distinction must be made between directly absorbable
(exchangeable) nutrient elements and those included in
mineral reserves.


Carbon and phosphorus depletion takes place mainly in solid
form (bedload and suspension), whereas nitrogen, total bases
and especially exchangeable bases migrate exclusive in
solution.

On the
other hand, under extensive cropping with fairly poor ground
cover, nutrient loss from erosion in kg/ha/yr amounts to 98 kg of
nitrogen, 57 kg of calcium, 39 kg of magnesium, and 29 kg of
phosphorus and potassium. To compensate these losses by
fertilizer applications would take 7 tonnes of fresh manure, 470
kg/ha of ammonia sulphate, 160 kg of superphosphate, 200 kg of
dolomite, and 60 kg/ha/yr of potassium chloride. Unsurprisingly,
then, the soils of southern Côte d'Ivoire are exhausted after
two years of traditional cropping, especially when removal by
harvesting and losses through drainage (800 mm per year) are also
added into the equation.

FIGURE
7 Selectivity, the danger of sheet erosion

SHEET
EROSION IS DANGEROUS, inasmuch as:

 it is hard to
recognize:

even if erosion
= 12 t/ha/yr = 1 mm of soil, it is less than natural soil
respiration;

1. The energy of
sheet runoff is too weak to move coarse particles: the
roughness of the soil surface reduces the speed and
competence of sheet runoff. Only organic matter, clay, silt
and their associated nutrients leave the plot; sand moves
over the soil surface and forms colluvial soil at the bottom
of the slope. If erosion speeds up, rills appear in which
erosion is no longer selective.

2. The
soil surface is generally richer in organic matter,
especially under forest.

THREE
EFFECTS

1. Accelerated
imbalance in nutrients (see Table 4)

2.
Accelerated imbalance in organic matter, leading to
degradation in structure, macroporosity and infiltration
capacity.

3. Soil
skeletonization through the relative accumulation of coarse
particles on the soil surface.

CONCLUSION

Sheet erosion speeds
up physical, biological and chemical degradation in the
surface horizons of cultivated land.

Stocking
(1986) later took Hudson's data from analyses of soil and water
collected on different plots in the 1960s, together with a map of
present-day land use in Zimbabwe, and calculated that each year
the country lost 10 million tonnes of nitrogen and 5 million
tonnes of phosphorus as a result of erosion.

Fortunately,
the nutrients lost to these plots are not definitively lost to
the country, but can be recovered on plots downstream, nourish
fish, or perhaps end up on rich alluvial or colluvial land,
though they may equally well provoke eutrophication.
Nevertheless, before launching a mineral fertilization project,
nutrient losses from erosion must first be halted, for they cause
a serious chemical imbalance in cultivated land (Roose 1980a;
Roose, Fauck and Pedro 1981).

PRODUCTION
LOSSES CAUSED BY RUNOFF

In hot
countries with temporary arid seasons, biomass production depends
on soil fertility, but still more on water availability when the
crop needs it.

Now, if the
water balance is calculated even roughly (Roose 1980a, or Somé
1989), it can be seen that in subequatorial zones, development of
25% runoff (a frequent rate on land under cereals, cassava and
other foodcrops) leads to a reduction in the volume of water
draining below the roots. This means that there is a certain
compensation between nutrient loss through runoff and through
drainage, although very few effects of runoff are seen on real
evapotranspiration, biomass production and crop yields.

Against
this, in semi-arid zones (mean annual rainfall less than 700 mm),
the same percentage of runoff actually observed under foodcrops
and cotton not only limits the possibility of drainage (and hence
of groundwater recharge), but also reduces real
evapotranspiration and hence the potential for biomass production
(Figures 8 and 9).

In the
day-to-day reality of arid zones, the depressive effect of runoff
on production is even more acute if water storage in the soil is
lowered by runoff at the start of the cropping cycle (delayed
seedling planting), or low during flowering (few ears fertilized)
or at the end of the cycle (grains imperfectly filled) due to
poor runoff and groundwater management (Nicou, Ouattara and Somé
1987). In Sudano-Sahelian zones the impact of runoff from the
first storms at the onset of the rainy season deserves emphasis.
These storms clear the surface of organic residues and animal
wastes that have collected throughout the dry season. Such losses
of organic matter lead to a considerable drop in the productivity
of land on the broad pediments of Sudano-Sahelian zones.

Another
generalized effect of runoff, whatever the climate, is to reduce
the concentration period of rainwater, increase peak discharge
(and hence sediment load and the scale of structural works), and
cause a reduction in the base discharge of rivers, particularly
in the dry season when water is needed for irrigation purposes.

Hydrologists,
who often look for catchment areas with heavy runoff after each
rainfall to feed lakes, reservoirs or towns, have a very
different viewpoint from agronomists, who look for better
infiltration and better actual evapotranspiration for better
plant production. Hydrologists and agronomists are, however, of
one mind in looking for clear water and the most even year-round
flow possible, in keeping with the principles of good management.
Even so, in arid zones, certain areas of the watershed may be
needed for water harvesting to ensure the growth of crops on
small areas (runoff farming) (Critchley, Reij and Seznec 1992).

FIGURE
8 Graph showing the water balance for the Sudano-Sahelian region
of Ouagadougou, Burkina Faso

TABLE 4Chart of the average
water balance for the Ouagadougou region, Sudano-Sahelian
savannah (cf. Roose 1980a)

Month

Rainfall (mm)

Potential evapotranspiration
(mm)

Runoff (mm)

Actual
evapotranspiration (mm)

Drainage (mm)

(1)

(2)

(1)

(2)

January

0

187

0

0

0

0

0

February

0

188

0

0

0

0

0

March

1

216

0

1

1

0

0

April

19

178

0

19

19

0

0

May

81

155

2

79

79

0

0

June

116

136

3

113

113

0

0

July

191

129

5

129

129

57

0

August

264

116

7

116

116

141

4

September

151

126

4

126

126

21

21

October

37

149

0

37

149

0

0

November

0

165

0

0

82

0

0

December

0

160

0

0

0

0

0

Total

860

1905

21

620

814

219

25

%

100

222

2,5

72

94,6

25,5

2,9

(1): gross(2): corrected

In
conclusion, runoff control has different consequences depending
on the water balance (Table 4). In high-rainfall areas, runoff
reduction leads to a slight improvement in the actual
evapotranspiration, but mainly to increased drainage and hence
increased risk of leaching and in the rate of flow when the river
is at its lowest. Agroforestry can be brought into play to
increase the actual evapotranspiration.

In
semi-arid zones (with less than 700 mm of mean annual rainfall)
runoff reduction increases the stored water available for the
actual evapotranspiration, and hence biomass production (and
yields).

LONG-TERM
EROSION-INDUCED REDUCTION IN SOIL PRODUCTION POTENTIAL

Runoff and
erosion can have an immediate deleterious effect on yields
of standing crops, but can also progressively modify the
physical, chemical and biological nature of the soil (through
selective erosion of the most fertile components) and reduce the
long-term potential of certain soils, especially thin soils (with
poor water- and fertilizer-storage capacities) and forest soils
(where fertility and biological activity are concentrated in the
surface horizons). It may be wondered whether the productivity of
these soils can be restored simply by stepping up the amount of
fertilizer used (cost of soil restoration).

In Nigeria,
Lal (1983) examined the impact of erosion on the productivity of
an alfisol at the IITA station near Ibadan, using three
approaches:

 On
24 erosion plots (125 m²) with 1, 5, 10 and 15% slopes, subject
to different treatments from 1971 to 1976, he measured different
levels of cumulative erosion and calculated the depressive effect
of erosion on the characteristics of the surface horizon,
particularly for carbon, nitrogen, assimilable phosphorus, pH and
total porosity. Multiple regressive analysis of the effect of
three soil properties on maize yields indicates that
erosion-induced changes in the soil have a significant effect on
yields.

Maize
yields (in t/ha) fall with cumulative erosion (E in t/ha), but
increase with the level of organic carbon (Co in %), total
porosity (Po in %) and infiltration capacity (Ic in cm/h). r is
the regression factor.

This
regression seems to indicate that erosion-induced reduction in
soil productivity can be countered primarily by adding organic
matter, and secondly through cropping techniques that improve
porosity (or water-storage capacity) and infiltration capacity.


Having obtained varying erosion levels on the same plots, Lal
then monitored maize yields during four cropping seasons
(1977-78), using the same treatment and average fertilizer rates
(40 + 80 N + 26 P, + 30 K per ha).

As
foreseen, the least erosion was on the plots with a 1% slope.
Despite this low rate, however, the best yields were not from
these 1% slopes but on plots with 5, 10 and 15% slopes.

FIGURE
9 Changes in the water balance under natural vegetation as a
function of climate (cf. Roose, Lelong and Colombani 1983)

NOTES ON FIGURE 9:

EFFECTS
OF RUNOFF CONTROL AS A FUNCTION OF THE WATER BALANCE

IN
SEMI-ARID ZONES

From the
agricultural viewpoint:


increase

in the
stored water available for plantsin the
actual evapotranspiration of plantsin the
biomass... and possibly in yields

From
the hydrological viewpoint:


reduction

in the
peak flows of riversin total
annual dischargein
suspended sediment


stabilization of gullies and rivers

IN
HUMID ZONES

From the
agricultural viewpoint:


modest increase in stored water available for crops,
actual evapotranspiration, biomass, and yields

On average,
maize yields fell by 0.26, 0.1, 0.08 and 0.1 t/ha per millimetre
of eroded soil on plots with 1, 5, 10 and 15% slopes,
respectively. The yield reduction rate for a 1% slope is thus two
to three times higher than on steeper slopes that are more
seriously eroded. This can be attributed to the fact that runoff
increases sharply on these flatter plots where infiltration
declines faster due to rain splash.

Apparently,
above a threshold of 4 mm (60 t/ha) of cumulative erosion in six
years, maize yields fall fast. This would give a tolerance rate
of roughly 10 t/ha/yr of erosion for this type of soil, although
it is difficult to generalize, since productivity reduction
rates, as a function of the decrease in soil depth, could have
been greater if there had been greater water stress during these
four farm seasons.

 Lal
tried to speed up his experiments by mechanically scouring the
surface of plots with a 1% slope just beside the erosion plots on
a ferralitic soil over a gravel sheet at about 25 cm (paleustalf)
that had been under bush fallow for 15 years.

The soil
was scoured to 0, 10 and 20 cm, and treated at three fertilizer
rates (N = 0, 60 and 120 kg/ha; and P = 0, 25 and 75 kg/ha) using
a split plot layout, with each treatment repeated three times,
and the maize grown without tillage.

Variance
analysis shows that scouring has a significant depressive effect
on plant height, leaf nutrient content, and the grain and biomass
yields. The surprising finding is that the effect of nitrogen
application is observable only on plots that have not been
scoured, implying that accelerated erosion can irreversibly
reduce the soil productivity of shallow soils (negative
interaction of erosion with mineral fertilization).

TABLE 5Three fertilization
levels on degraded soils

Fertilization

N

P

K

Mg

Zn

Mo

Lime (kg/ha)

F0

0

0

0

0

0

0

0

F1

110

50

0

20

5

1

0

F2

220

450

250

100

10

2

3500

Moreover,
the reduction rate in terms of grain (0.13 and 0.09 l/ha/cm of
scoured soil) and straw (0.16 and 0.12 t/ha/cm) is much higher
for the first centimetres of topsoil scoured, and would be even
more marked if less than 10 cm had been scoured at a time.

Nitrogen
and phosphate applications had positive effects on N and P
content in the leaves of cultivated plants. If grain yields did
not rise in economic terms, this was because of other limiting
factors, particularly the porosity, structure and water-storage
capacity of deep horizons now exposed to rain splash.

Comparison
of productivity losses from artificial scouring (0.013 t/ha/mm)
and natural erosion (0.26 t/ha/mm) on the same shallow soil shows
that the effects of natural sheet erosion are 20 times more
serious than simple mechanical scouring, since sheet erosion
selectively removes the most fertile elements: organic matter,
clays and loams, and the most soluble nutrients.

Lal
concludes that the studies on "scoured plots" gave only
relative indications of the impact of erosion on soil
productivity, especially on shallow soils (Figure 10).

Another
example has been given on a recently cleared oxisol (tropeptic
eutrustox, kaolinitic clayey) on the island of Oahu (Hawaii)
(El-Swaify, Dangler and Armstrong 1982). Preliminary studies
showed that this soil had a strong concentration of fertility in
the first ten centimetres as well as in a compact layer at about
35 cm. It was then decided to assess the harmful impact of
scouring the first ten centimetres and exposing the subsoil with
its unfavourable physical characteristics. The three treatments
(scouring of 0, 10 and 35 cm) were restored with three levels of
fertilization (0, 50 and 100% of requirements to attain peak
production) (Figure 11).

The results
showed that this oxisol has a much higher potential (11 t/ha)
than the Ibadan alfisol and that fertilization (especially N and
P) has a decided influence in almost all situations. Even so,
yields fall very sharply when 35 cm of soil is removed, probably
because the rooting system develops poorly in this extreme case.

Without
fertilizer (Fo), yields fell by half for a 10 cm scouring, and by
90% when the whole humiferous horizon was removed. There appears
to be a threshold beyond which yields fall sharply even if large
doses of fertilizer are applied (110 N + 50 P) (Table 5).

The cost of
restoring severely scoured soil becomes uneconomic (220 N + 450 P
+ 250 K + 3500 CaO), for the physical properties of the subsoil
are unsuitable for root growth and the soil's phosphorus fixation
rate is very high. The economic impact of erosion is particularly
marked when restoration of the subsoil requires major phosphate
and lime inputs, a syndrome very frequent in tropical soils.

FIGURE
11 Maize production as a function of the level of mechanical
scouring and mineral fertilization with a view to restoring
productivity (2nd harvest) (cf. El-Swaify, Dangler and Armstrong
1982)

These two
experiments on tropical soils with very different productivity
demonstrate clearly how sheet erosion, although hard to see, can
have a serious long-term effect on the productive capacity of
soils. Even if it is modest in annual terms, this depressive
effect is cumulative, eventually thrusting itself into the
limelight when certain properties of the soil pass tolerance
thresholds:

A soil
degraded through sheet erosion is a tired soil and barely reacts
to applications of mineral fertilizer. This is what happens with
shallow ferralitic soils on ironstone or gravel sheet, and soils
with compacted horizons close to the surface.

Even so,
not all soils are non-renewable natural resources. In the section
on "Restoring Soils and Rehabilitating Land" in Chapter
2, it was seen that if a series of six rules is respected (and
not simply the application of mineral fertilizers), the fertility
of a good number of sufficiently deep soils can quickly be
restored. However, the cost of such restoration rises the longer
the delay in protecting the soil: thus the soil has to be tilled
in depth, fermented organic matter, fertilizers and conditioners
have to be applied, the induced porosity has to be taken over by
an abundant biomass... and the soil has to be protected against
runoff.

Lastly, in
the very special case of old ferralitic soils that are acid and
completely desaturated (ultisols), it might seem best to speed up
their erosion in order to improve their productivity. However,
the price would be high, for one must take account of the impact
of huge quantities of sterile matter that would clutter up the
richer plains, and also envisage a major investment in order to
restore fertility to the rejuvenated soils. When experimental
bench terraces were built on the Rwanda hills, nothing grew on
these soils altered down to Horizon B without a huge application
of manure (30 t/ha) combined with liming (3 t/ha every two years)
and supplementary mineral dressings (50 N + 50 P + 50 K) (Rutunga
1992).

So erosion
affects the production potential of a soil. In the case of a
desaturated ferralitic soil (e.g., alfisols), if it has been
eroded it can no longer store water and nutrients and supply them
to crops as and when needed. It has also lost some of the
biogenic components of the topsoil, and so micro-organisms are
inefficient or slower at recycling the nutrients contained in the
soil. Lastly, rooting is usually insufficient in the subsoil when
the topsoil has been eroded.